Exhorting readers to "abandon common sense," Comins (What if the Moon Didn't Exist?) proceeds to reveal that there are actually 13 (not 12) zodiacal constellations, that the powers-that-be cannot possibly send us into a black hole for misbehaving and that the asteroid belt in The Empire Strikes Back is implausibly dense. Debunking silly, frightening and grandiose beliefs, the University of Maine professor of physics and astronomy is reassuring and engaging. Illus. (Aug.)